1. Field of the Invention
The present invention relates generally to stepping motors for use in business machines such as printers and automotive electric equipment, and more particularly to a permanent-magnet, built-up stepping motor.
2. Description of the Prior Art
In the manufacture of gearmotors, a motor unit and a reduction gear must be assembled neatly without mismatches between the reduction gear and bearings by which a rotor is supported. An example of such gearmotors disclosed in Japanese Utility Model Publication No. 63-5419 has a built-up construction composed of various component parts assembled successively one above another in a deep case or housing.
Japanese Utility Model Laid-open Publication No. 62-7761 exemplifies a geared stepping motor composed of a stepping motor unit and a reduction gear having a train of gears disposed between two opposed support plates. The stepping motor unit and the reduction gear are assembled as separate units and subsequently they are assembled together.
The gearmotor disclosed in Japanese Utility Model Publication No. 63-5419 includes a rotor shaft formed integrally with the deep housing and disposed co-axially in the housing. A stator bobbin having stator windings, a rotor with a pinion, and a reduction gear are stacked in succession about the rotor shaft. Since all the component parts are disposed in the deep housing from the above, manipulation of these component parts involves various limitations which will make it difficult to automate the assembling operation. Furthermore, the integral formation of the rotor shaft and the housing requires a precision working which per se involves various difficulties. With this integral rotor shaft, an error-free mass production of the geared stepping motor is practically impossible. Additionally, the rotor journaled on the rotor shaft must be hollow in shape and fitted accurately with the rotor shaft. This arrangement needs an increased number of manufacturing processes.
The geared stepping motor disclosed in the Japanese Utility Model Laid-open Publication No. 62-7761 has a construction other than the built-up construction and hence is not suited for automated assembling on a transfer machine. Since the geared stepping motor has a relatively large number of component parts and needs a deep housing, building up of the component parts in a stacked condition is difficult to achieve.
The conventional stepping motors include a stator and a rotor assembled in a housing, with the rotor supported by bearings mounted on the housing. As disclosed in Japanese Utility Model Laid-open Publication No. 62-14950, the bearings may comprise oil-impregnated bearings, oilless bearings, oilless metals, ball bearings, etc.
Partly because the bearings are provided on the stepping motor unit, and partly because the stepping motor unit and the reduction gear are formed as separate units before they are assembled together, the overall size of the assembled geared stepping motor is considerably large, accordingly.
In the conventional stepping motors, a stator and a rotor disposed in a housing are assembled in the built-up construction by means of screw fasteners. With this screwed assembly, the stator cores are prevented from interfering with each other. To achieve an automated assembling, stator cores having the so-called "claw-pole" construction are assembled layerwise without using screw fasteners. Upon energization of a stator coil, the assembled stator cores tend to oscillate in resonance with the excited stator coil, thereby generating unpleasant noises.
Furthermore, the conventional stepping motors have terminals provided on ends of a stator coil wound on and around a coil bobbin. After winding, the stator coil is bound with a bind tape or a synthetic resin against loosening and subsequently is connected with terminals for connection with an external connector. The terminals are pulled to the outside of the motor. The foregoing construction has a drawback that the stator coil is likely to be damaged or broken when it is pulled to connect the terminals to the external connector. The terminals may be integrally molded with the bobbin. In this instance however, the stator coil is still subjected to pulling forces when the terminals are connected with or released from the external connector. Furthermore, the integral formation of the terminals and the bobbin lowers the mechanical strength of the bobbin and makes it difficult to assemble the built-up stepping motor.